The present invention relates to a method for the spatially resolved determination of the series resistance of a semiconductor structure as recited in the preamble of Claim 1, the semiconductor structure being a solar cell or a preliminary stage in the production of a solar cell, including at least one pn junction and contacts for electrical contacting.
The series resistance is an essential quantity for characterizing a solar cell, because a high series resistance typically causes a reduction of the efficiency of the solar cell. The total series resistance of a solar cell is comprised of a plurality of portions; for example, the cross-conductor resistance of a metallic contacting structure, the cross-conductor resistance of a doping layer such as an emitter layer, and/or the contact resistance between the metallic contact structure and the doping layer can contribute essentially to the total series resistance.
In order to characterize a solar cell, as well as for process control in the production of a solar cell, it is desirable to determine the series resistance of the solar cell in spatially resolved fashion, i.e. to determine the local series resistance at each of a plurality of locations. The distribution of the local series resistances enables inferences to be drawn concerning locally inhomogenous process conditions, or faulty elements such as interrupted metallization structures.
A number of measurement methods are known for the spatially resolved determination of the series resistance; in particular, the spatially resolved measurement of luminescence radiation produced in the solar cell is suitable for such measurement methods. It is already known to use a CCD camera to measure, in spatially resolved fashion, the luminescence radiation emanating from a surface of the solar cell, and on this basis to determine the series resistance in spatially resolved fashion:
In T. Trupke, E. Pink, R. A. Bardos, and M. D. Abbott, “Spatially resolved series resistance of silicon solar cells obtained from luminescence imaging,” Applied Physics Letters 90, 093506 (2007), a method is described in which luminescence radiation is produced by illuminating the solar cell in a known manner, and this so-called photoluminescence radiation is measured in spatially resolved fashion using a CCD camera. Here, two images of the photoluminescence radiation are taken under different measurement conditions; under one measurement condition A, open-circuit conditions are present (i.e., there is no flow of current between the contacts), and under a measurement condition B current is drawn from the solar cell. In addition, at least one third image of the photoluminescence radiation under short-circuit conditions is required in order to clear the measurement values of the two previously noted images.
From the measurement image taken under measurement condition A, spatially resolved calibration parameters Ci are determined, one calibration parameter being determined for each location i. Using these calibration parameters, the local intensities of the luminescence radiation, measured under measurement condition B, are converted into a voltage present locally at the respective location of the solar cell.
Under the assumption that a uniform value for a dark saturation current can be assumed for the entire solar cell, the determination of the local series resistances is possible using the known one-diode model as an approximation for the modeling of the local electrical properties of the solar cell.
In typical commercially produced solar cells, in particular solar cells made of multicrystalline silicon, however, a locally homogenous dark saturation current cannot be assumed. For the quantitative determination of the local series resistances in such solar cells, a spatially resolved determination of the dark saturation current is therefore additionally required.
Standardly, for this purpose further measurements of the photoluminescence are necessary under different measurement conditions, such as exposing the solar cell to electromagnetic radiation having different wavelengths.